<!DOCTYPE html>
<html>

<head>
  <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
  <title>系统时间</title>
</head>

<body>
<script>
  Date.prototype.format = function (fmt) {
    var o = {
      "y+": this.getFullYear, //年
      "M+": this.getMonth() + 1, //月份
      "d+": this.getDate(), //日
      "h+": this.getHours(), //小时
      "m+": this.getMinutes(), //分
      "s+": this.getSeconds() //秒
    };
    if (/(y+)/.test(fmt)) fmt = fmt.replace(RegExp.$1, (this.getFullYear() + "").substr(4 - RegExp.$1.length));
    for (var k in o)
      if (new RegExp("(" + k + ")").test(fmt)) fmt = fmt.replace(RegExp.$1, (RegExp.$1.length == 1) ? (o[k]) : (("00" + o[k]).substr(("" + o[k]).length)));
    return fmt;
  }
  setInterval("document.getElementById('dateTime').innerHTML = (new Date()).format('yyyy-MM-dd hh:mm:ss');", 1000);
</script>

<div id="dateTime"></div>
</body>